In a communication system, signals of the transmitting end reach the receiving end after being transmitted via channels. In this process, the waveforms of the signals have been seriously distorted. At the receiving end, transmitted data cannot be obtained if no processing is performed. In a practical receiver, clock and data recovery (CDR) is needed, wherein, the object of clock recovery (CR) is to generate a clock signal at the present locality, the frequency and phase of the clock signal being in consistence with the frequency and phase of the symbol changes of a received signal.
FIGS. 1 and 2 are schematic diagrams of a coherent optical communication system. At the receiving end, the received optical signals are mixed with a local oscillator laser, a coherent detector converts optical signals into electrical signals, and an analog-digital converter (ADC) performs sampling and quantization on the electrical signals to convert them into digital signals.
In FIG. 1, the output of a voltage controlled oscillator (VCO) is used to drive the ADC to perform the sampling, and a coherent optical communication system often requires that the sampling rate of an ADC is two times of the symbol rate. The output of the VCO is a local clock signal. The phase (frequency) of an output signal of the VCO is controlled by an externally applied voltage. And the output of the phase detector is applied to the VCO to control the phase of the output signal of the VCO, thereby controlling the sampling phase of the ADC. In this embodiment, the clock recovery module comprises a phase detector and a voltage controlled oscillator. As waveforms of signals will be distorted due to dispersion when optical signals are transmitted via optical channel, and the waveforms of the signals will be seriously distorted if the dispersion is very large and the clock recovery will fail, a dispersion compensator is arranged after the ADC, which compensates for most of the accumulated dispersion, making the clock recovery succeeded. Refer to document [2] for the arrangement of the dispersion compensator. The dispersion compensated signals enter into the clock recovery module and into the data recovery module. The clock recovery module (the frame in dotted lines) comprises a phase detector and a voltage controlled oscillator. And the data recovery module comprises a series of digital signal processing (such as adaptive equalization, and phase recovery, etc.), which finally recovers the transmitted data. Correct data recovery requires that the sampling frequency and phase of the ADC are in consistence with the frequency and phase of the symbol changes of a signal, which is ensured by the clock recovery.
FIG. 2 is another embodiment of the coherent optical communication system. Different from FIG. 1, in the coherent optical communication system shown in FIG. 2, the ADC is not drove by a VCO but by a free oscillator. After the ADC, the digital signals are resampled. As the resampling may equivalently introduce arbitrary delay, and the size of the introduced delay is controlled by the output signals of the phase detector, it may be equivalent to adjustment of a sampling moment. The clock recovery module in this embodiment comprises a phase detector and a resampler.
Both embodiments shown in FIGS. 1 and 2 use a phase detector, which is a key module in clock recovery. It indicates whether the current sampling phase (moment) advances or lags behind the phase of symbol changes, and according to this, the sampling phase may be directionally adjusted. In FIG. 1, the phase of the output signals of the VCO may be adjusted according to the phase difference, thereby directly adjusting the sampling moment of the ADC; and in FIG. 2, the delay introduced by the resampling module may be adjusted according to the phase difference, thereby equivalently adjusting the sampling moment. A commonly-used method of a phase detector is the Gardner method, which calculates respectively the phase differences of the I component and Q component of a received signal.
However, in the implementation of the present invention, the inventors found that certain limitations exist in the Gardner method. On the one hand, in a coherent optical communication system, there exists a certain frequency difference between the local oscillator laser of the receiving end and the laser of the transmitting end, and both of them have a respective line width. In case of no phase recovery, the actual I component and Q component cannot be obtained from a received signal. Therefore, the Gardner method will be invalid when there is a relatively large frequency difference or line width. And on the other hand, an optical communication system of a higher capacity of the next generation will employ Nyquist signals of higher spectral efficiency. Nyquist signals are signals with strictly-limited bandwidths, and their spectra are strictly limited to an interval [−B(1+α)/2, B(1+α)/2]. Where, B is a symbol rate, and α is referred to as a roll-off factor. For a Nyquist signal of a spectral width close to the symbol rate (with a very small roll-off factor α), the Gardner method will be invalid even if there is no frequency difference.
It should be noted that the above description of the background art is merely provided for clear and complete explanation of the present invention and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background art of the present invention.